Our overall goal is to understand the mechanisms by which dividing cancer and normal cells are killed, or escape death, when treated with diverse anti-mitotic drugs. We seek understanding both at the level of cell population behaviors, and the molecular mechanisms that give rise to those behaviors. We are especially interested in understanding how different classes of anti-mitotic drug differ in their ability to kill cancer cells, and using that information to design better future drugs. We will address these goals in 4 specific aims: Aim 1. How do drugs with different anti-mitotic mechanisms differ in their ability to kill cancer cells at the level of cell population behavior? We will use microscopy with fluorescent reporters for key molecular events to measure single-cell responses to four anti-mitotic drugs across a panel of cell lines chosen to vary in apoptosis sensitivity. We will also test the effects of blocking different aspects ofthe drug response. Aim 2. Elucidate the molecular mechanism of cell death during mitotic arrest. To determine how the intrinsic apoptosis pathway is activated by prolonged mitotic arrest we will test candidate regulators of apoptosis, and pursue an unbiased biochemical approach. We will also Identify an alternative cell death pathway cells use when they cannot escape mitotic arrest, and apoptosis is blocked. Aim 3. Develop small molecule inhibitors of mitotic exit that work independent of the SAC. Our preliminary data suggest that novel anti-mitotic drugs with this mechanism would kill cancer cells more effectively than current drugs. We will test this concept, and identify druggable targets in the mitotic exit pathway, by small molecule screening in Core B using a cell-based assay, followed by identification of protein targets of hits. Aim 4. Compare drug responses in mouse tumors to those seen in cell culture. Drug response mechanisms elucidated in cell culture in aims 1-3 may not translate to real tumors. We will test whether cells that die during mitotic arrest in mouse tumors cause bystander killing of non-dividing tumor cells, which could explain why some tumors with low mitotic index can be treated with anti-mitotic drugs. Working with Core C we will probe responses to anti-mitotic drugs in mouse tumors at the single-cell level by intravital imaging.